Internal combustion engines are energy conversion mechanisms used by the great majority of motor vehicles and basically comprise two main parts—one or more cylinder heads and the engine block. Below the cylinder head(s) are located the combustion chambers, and the cylinders and the crankshaft assembly are located in the engine block.
The crankshaft is a fundamental component in the functioning of combustion engines, and is responsible for converting the energy generated by combustion of the air/fuel mixture into torque. The crankshaft, which is mounted in the block by supporting bearings, is usually of cast carbon steel and has connecting rods coupled to the crank pins and their geometry is reminiscent of a number of cranks placed side by side.
Among the components which help to mount the crankshaft in the engine block are bushes and thrust washers, the washers being responsible for withstanding axial loads generated by the gearing or transmission system.
Thrust washers are generally located in one or more bearing positions, and basically comprise two half-moons or two washers positioned at the top and, alternately, also at the bottom of the bearing. These washers act in such a way as to withstand possible axial forces transmitted by the crankshaft, so that when they are exposed to an axial load the force generated does not act directly on the engine block because the washer provides a supporting surface for the load, preventing wear on the block.
As may be seen in FIG. 1, the thrust washer is provided with a body, substantially of a semi-circumferential shape, and has two surfaces. The planar contact surface is associated with the engine block, and there is no relative movement between the washer and the block, while the slip surface remains in contact with the crankshaft, resulting in relative movement between the washer and the crankshaft. Depending upon the applied load, the relative movement places a high load on the contact between the slip surface and the crankshaft, the slip surface being responsible for supporting the axial load produced and, additionally, the area where oil channels are present.
The oil channels are fundamental to the proper functioning of thrust washers and ensure that the surface is lubricated, preventing accelerated/excessive wear on the parts in relative movement. These channels are filled with oil so that the oil is distributed over the surface through the pull generated by the rotational movement.
Both Otto and diesel engines are subject to axial loads. Furthermore, these loads are becoming ever greater, mainly in engines provided with automatic or automated transmissions. These transmissions reduce fuel consumption in motor vehicles, but they increase the applied loads, with the result that the thrust washers currently in use cannot withstand the forces and ultimately become worn or suffer other deficiencies.
In order for the washer to support the loads generated, an oil film has to be formed between the slip surface of the washer and the crankshaft. This oil film must maintain a minimum thickness, and it is created through a phenomenon known as hydrodynamic support. This hydrodynamic support depends on two main factors—the shape of the surface and the relative speed between the faces.
In order to achieve sufficient hydrodynamic support, a specific relative speed, and a surface to provide for that condition, have to be guaranteed. As the speed varies as a function of engine rotation, the surface can be modified in such a way as to achieve sufficient support to support high axial loads and forces.
In the state of the art there are flat washers and washers with hydrodynamic profiles, and they may be convex or ramp-and-pad.
Washers having a flat slip surface, as the name suggests, do not have any geometry on the slip surface, and only include oil channels. These washers have low resistance to binding and are only suitable for engines with low loads.
Convex washers comprise a curvilinear slip surface. They have low resistance to binding because of the lack of flat sections. It should be noted that a flat section is essential in order to maintain hydrodynamic support.
Ramp-and-pad washers, meanwhile, comprise at least one ramp with a constant inclination, at least one flat horizontal section and at least one oil channel. These washers ensure a high level of hydrodynamic support as compared to flat or convex washers.
Although ramp-and-pad washers have better performance, the connection between the ramp section and the horizontal section represents a sudden change in geometry. This sudden change point gives rise to an inflection point or vertex reducing hydrodynamic support.